Concern for increased bacterial resistance to available antibiotics has grown extensively in recent years. While much of the concern relates to acquired resistance attributed at least in part to the widespread use of broad spectrum antibiotics by medical practitioners, there are also some concerns about antibiotic resistance stemming from the use of bacteria for food and agricultural purposes which have acquired antibiotic resistance genes. For example, the presence and distribution of tetracycline resistance genes are common in microorganisms isolated from the environment, animals and humans including children. In some cases, antibiotic resistance may be intrinsic to a particular species—for example, due to specific cell membrane properties. Antibiotic resistance may also be acquired through mutation of bacterial genes, and it may be acquired from resistant bacteria in the environment by gene transfer.
Use of any bacterium that possesses or has acquired antibiotic resistance in food processing or agricultural production poses a potential, theoretical risk of transfer of the resistance fostering genes to other bacteria in the food, the gastrointestinal tract (GIT) of a person or animal after consumption of the food, or the environment, at some point before or after consumption. Examples of such a bacterium can be found among the Bifidobacterium spp., for example, Bifidobacterium animalis such as Bifidobacterium animalis subsp. lactis (also referred to as Bifidobacterium animalis subsp. lactis subsp. nov., previously regarded as Bifidobacterium lactis and sometimes referred to herein as such). One such strain, B. animalis subsp. lactis strain NCC 2818, CNCM I-3446, is commercially available, has been used for over 20 years as an additive to food products and is generally regarded as safeIt has recently been discovered that this bacterium, in common with certain other gram(−) and gram(+) bacteria of human and animal origin has a tetracycline-resistance gene, tetW, present. Although tetracycline resistance transference is theoretically possible, it has so far not been possible to demonstrate this even under laboratory conditions. It is, however, known from cloning experiments that tetW from this strain is active in gram(+) bacteria although not in gram(−) bacteria.
However, to eliminate any risk of unintended transfer of this gene, there is a need for methods for its removal from this strain. There is also a need for a variant of the strain with reduced resistance to tetracycline.